Process for upgrading methane to higher hydrocarbons

ABSTRACT

There is provided a process for the direct partial oxidation of methane with oxygen, whereby hydrocarbons having at least two carbon atoms are produced. The catalyst used in this reaction is a cadmium-manganese oxide catalyst.

BACKGROUND

There is provided a process for the direct partial oxidation of methanewith oxygen, whereby hydrocarbons having at least two carbon atoms areproduced. The catalyst used in this reaction is a cadmium-manganeseoxide catalyst.

Natural gas is an abundant fossil fuel resource. Recent estimates placesworldwide natural gas reserves at about 35×10⁴ standard cubic feet,corresponding to the energy equivalent of about 637 million barrels ofoil.

The composition of natural gas at the wellhead varies but the majorhydrocarbon present is methane. For example the methane content ofnatural gas may vary within the range of from about 40 to 95 vol. %.Other constituents of natural gas may include ethane, propane, butanes,pentane (and heavier hydrocarbons), hydrogen sulfide, carbon dioxide,helium and nitrogen.

Natural gas is classified as dry or wet depending upon the amount ofcondensable hydrocarbons contained in it. Condensable hydrocarbonsgenerally comprise C₃ + hydrocarbons although some ethane may beincluded. Gas conditioning is required to alter the composition ofwellhead gas, processing facilities usually being located in or near theproduction fields. Conventional processing of wellhead natural gasyields processed natural gas containing at least a major amount ofmethane.

Processed natural gas, consisting essentially of methane, (typically85-95 volume percent) may be directly used as clean burning gaseous fuelfor industrial heat and power plants, for production of electricity, andto fire kilns in the cement and steel industries. It is also useful as achemicals feedstock, but large-scale use for this purpose is largelylimited to conversion to synthesis gas which in turn is used for themanufacture of methanol and ammonia. It is notable that for theforegoing uses no significant refining is required except for thoseinstances in which the wellhead-produced gas is sour, i.e., it containsexcessive amounts of hydrogen sulfide. Natural gas, however, hasessentially no value as a portable fuel at the present time. In liquidform, it has a density of 0.415 and a boiling point of minus 162° C.Thus, it is not readily adaptable to transport as a liquid except formarine transport in very large tanks with a low surface to volume ratio,in which unique instance the cargo itself acts as refrigerant, and thevolatilized methane serves as fuel to power the transport vessel.Large-scale use of natural gas often requires a sophisticated andextensive pipeline system.

A significant portion of the known natural gas reserves is associatedwith fields found in remote, difficulty accessible regions. For many ofthese remote fields, pipelining to bring the gas to potential users isnot economically feasible.

Indirectly converting methane to methanol by steam-reforming to producesynthesis gas as a first step, followed by catalytic synthesis ofmethanol is a well-known process. Aside from the technical complexityand the high cost of this two-step, indirect synthesis, the methanolproduct has a very limited market and does not appear to offer apractical way to utilize natural gas from remote fields. The Mobil OilProcess, developed in the last decade provides an effective means forcatalytically converting methanol to gasoline, e.g. as described in U.S.Pat. No. 3,894,107 to Butter et al. Although the market for gasoline ishuge compared with the market for methanol, and although this process iscurrently used in New Zealand, it is complex and its viability appearsto be limited to situations in which the cost for supplying an alternatesource of gasoline is exceptionally high. There evidently remains a needfor other ways to convert natural gas to higher valued and/or morereadily transportable products.

One approach to utilizing the methane in natural gas is to convert it tohigher hydrocarbons (e.g. C₂ H₆ ; C₂ H₄ ; C₃ H₈ ; C₃ H₆ . . . ); thesehave greater value for use in the manufacture of chemicals or liquidfuels. For example, conversion of methane to ethane or ethylene,followed by reaction over a zeolite catalyst can provide a route togasoline production that entails fewer steps than the indirect route viamethanol synthesis described above. Unfortunately, the thermalconversion of methane to ethane is a thermodynamically unfavorableprocess (ΔG°>+8 kcal/mol CH₄) throughout the range from 300-1500K. Theupgrading reactions explored here are oxidative conversions of methaneto higher hydrocarbons, as exemplified in the following equations.

    CH.sub.4 +0.25O.sub.2 →0.5C.sub.2 H.sub.6 +0.5H.sub.2 O

    CH.sub.4 +0.50O.sub.2 →0.5C.sub.2 H.sub.4 +1.0H.sub.2 O

Analogous reactions include those converting methane to C₃, C₄, . . .and higher hydrocarbons. These oxidation processes have very favorablefree energy changes (ΔG°<-19 kcal/mol CH₄) throughout the temperaturerange of 300-1000K. The oxidation reactions are commonly performed inthe presence of a catalyst. The use of the catalyst allows the reactionto occur under conditions where there is essentially no thermal reactionbetween methane and oxygen. The catalyst can also favorably influencethe selectivity of the oxidation reaction to minimize the extent ofover-oxidation to CO and CO₂.

SUMMARY

There is provided a process for synthesizing one or more hydrocarbonshaving at least two carbon atoms by the direct partial oxidation ofmethane, said process comprising contacting a mixture of methane andoxygen with a cadmium-manganese oxide catalyst under sufficientconversion conditions. After this conversion, the one or morehydrocarbons may be recovered.

The cadmium-manganese oxide composition provides superior selectivity toC₂ + products (i.e. products having 2 or more carbon atoms) vs. thatobtained with simple manganese oxides or that of manganese incombination with other divalent ions such as magnesium.

EMBODIMENTS

The cadmium-manganese oxide catalyst comprises a mixture of cadmium andmanganese in oxide form. One method of preparing this catalyst is bycalcining a suitable physical mixture of cadmium and manganesecompounds, for example a mixture of Cd(OH)₂ and Mn₂ O₃. Alternatively,the catalyst may be prepared by impregnating a solution of one compoundonto the solid form of another compound, for example impregnating asolution of Cd(NO)₂ onto a manganese oxide, followed by a suitablecalcination. Similarly, solutions of both cadmium and manganesecompounds can be coevaporated and/or precipitated, and the resultingmaterials calcined. Metal compounds that might be employed in thesepreparations include the oxides, hydroxides, acetates, carbonates,sulfates, nitrates, nitrites, or halides of manganese or cadmium, aswell as metal complexes of manganese and/or cadmium. Catalyticperformance of the resulting material may be a function of thecomposition (in terms of wt-% Cd or Mn) as well as the manner ofcatalyst preparation. The catalyst may contain, for example, at least 10weight percent of Cd and at least 10 weight percent of Mn, based uponthe total weight of Cd and Mn in the cadmium-manganese oxide.

In the practice of the present invention, it is preferred to use a dualflow system, i.e., a sYstem in which the methane and the oxygen or airare kept separate until mixed just prior to being introduced into thereactor. However, if desired, the oxygen and methane may be premixed andstored together prior to the reaction. The preferred dual flow systemminimizes the risk of fire or explosion. The methane feed for thepresent reaction may be provided by pure methane or by a methanecontaining gas, e.g., containing at least 50 percent by weight methane.An example of a methane feed is natural gas.

Air may be used instead of oxygen; inert diluents such as nitrogen,argon, helium, steam or CO₂ may also be cofed. The gas comprising themethane may be derived from processed natural gas. In the system, theamount of oxygen is controlled so as to prepare a reaction mixture wherethe volume ratio of methane to oxygen is within the range of 0.1-100:1,more preferably in the range of 1-50:1, even more preferably in therange of 1-10:1. The operating pressure for the reactants (methane andoxygen) may be within the range of 0.1 to 30 atmospheres, preferablywithin the range of 0.5-5 atm. The flow rate of the feed gas over thecatalyst may be expressed as the volumetric gas flow rate at ambienttemperature and pressure divided by the volume of catalyst, giving theGas Hourly Space Velocity (GHSV) in units of h⁻¹. Preferred GHSV iswithin the range of 10-100,000 h⁻¹, more preferably within the range of50-50,000 h⁻¹. The GHSV may be chosen to maximize the selectivity tohigher hydrocarbon products, to maximize the yield of higher hydrocarbonproducts, or to maximize the conversion of either methane or oxygenreactant.

The temperature in the reaction zone may be from about 300° C. to 1200°C., and preferably from about 500° C. to 1000° C., more preferably from600° C. to 900° C.

EXAMPLE

The following terms are defined. Methane conversion: the percentage ofcarbon atoms in the feed converted to other products. C₂ + selectivity:percentage of carbon atoms derived from converted methane which ends upas C₂ H₆, C₂ C₄, C₃ H₈, C₃ H₆, . . . (i.e. higher hydrocarbons,non-CO_(x)). C₂ + yield: the percentage of total feed carbon which endsup as higher hydrocarbons (i.e. conversion X selectivity).

Catalyst A was prepared by high temperature reaction (900° C.) of amixture of Cd(OH)₂ and Mn₂ O₃ ; composition of the final material waschecked by elemental analysis. Catalyst B was prepared by impregnatingMn₂ O₃ with an aqueous solution of Cd(NO₃)₂. The impregnated solid wasdried at 150° C., followed by intervals of calcination at 350° C. and900° C. A magnesium-manganese oxide catalyst was prepared by hightemperature reaction of the solids MgO and Mn₃ O₄. Unmodified oxides,Mn₂ O₃ and Mn₃ O₄, and were obtained from commercial sources and wereused as supplied. Reactions were run in a 14 mm ID×140 mm length quartzreactor; 0.1 g of 230/325 mesh catalyst was mixed with 4 g of 50 meshquartz chips and loaded into the reactor along with additional pre- andpost-beds of quartz sufficient to fill the reactor volume. Feed gaseswere delivered at atmospheric pressure from mass flow controllers. Thetemperature in the catalyst bed was measured through a quartz thermowelland ranged from 750°-760° C. The catalysts were conditioned in thereactor at 750° C. for 1 h under O₂ (25 cc/min) prior to starting thefeed of 25 mol% CH₄, 5 mol% O₂, 70 mol% N₂ (space velocities given inthe table are for gas flow rates measured at ambient conditions). Waterproduced in the reaction was condensed from the effluent into a chilledtrap (-3° C.) and the product gas was analyzed on a Carle refinery gasanalyzer. In the absence of a catalyst, there is no reaction of methaneunder the specified conditions.

Table I gives the CH₄ conversion and C₂ + selectivity for the examinedcatalyst; note the given data allows for comparison of the selectivityat essentially constant methane conversion. The cited data for all theseexamples were taken after the systems had reached stable operation (2-18h). The C₂ + selectivity of the cadmium-promoted materials (≧32.5%) issuperior to the unmodified manganese oxides (≦19.3%), and likewise issuperior to the magnesium modified manganese oxide (0.0%).

                  TABLE I                                                         ______________________________________                                        Comparison of Oxidative Coupling Results for                                  Cadmium-Promoted Manganese Oxides.                                                                         CH.sub.4                                                                              C.sub.2 +                                                 Space Velocity                                                                            Conv.   Sel.                                     Catalyst         (cc/m/g cat)                                                                              (% C)   (% C)                                    ______________________________________                                        Catalyst A Cd/Mn/O.sub.x                                                                        500        11.6    62.5                                     (35.2 wt % Cd, 39.0 wt % Mn)                                                  Catalyst B Cd/Mn/O.sub.x                                                                       1000        10.4    32.5                                     (21.3 wt % Cd, 53.2 wt % Mn)                                                  Mn.sub.2 O.sub.3 1000        9.8     19.3                                     Mn.sub.3 O.sub.4 1000        9.7      0.0                                     Mg/Mn/O.sub.x     500        8.0      0.0                                     (12.6 wt % Mg, 54.4 wt % Mn)                                                  ______________________________________                                    

What is claimed is:
 1. A process for synthesizing one or morehydrocarbons having at least two carbon atoms by the direct partialoxidation of methane, said process comprising contacting a mixture ofmethane and oxygen with a cadmium-manganese oxide catalyst underconditions sufficient to convert methane to said one or morehydrocarbons having at least two carbon atoms.
 2. A process according toclaim 1, wherein said cadmium-manganese oxide catalyst is prepared bycalcining a mixture of Cd(OH)₂ and Mn₂ O₃.
 3. A process according toclaim 1, wherein said conversion conditions include a temperature offrom about 300° C. to about 1200° C. and a reactant partial pressure offrom about 0.1 atm to about 30 atm.
 4. A process according to claim 1,wherein said mixture of methane and oxygen has a volume ratio of methaneto oxygen of 0.1-100:1.
 5. A process according to claim 1, wherein saidmixture of methane and oxygen is provided by a mixture of natural gasand air.
 6. A process according to claim 3, wherein said conversionconditions include a Gas Hourly Space Velocity of from 10 to 100,000h⁻¹.
 7. A process according to claim 4, wherein said conversionconditions include a temperature of from 600° C. to 900° C., a reactantpartial pressure of from 0.5 to 5 atm and a Gas Hourly Space Velocity offrom 50 to 50,000 n⁻¹.
 8. A process according to claim 7, wherein saidmixture of methane and oxygen has a volume ratio of methane to oxygen of1-10:1.